June 16 (Reuters) - British prompt gas prices rose 18 percent on Friday due to supply shortfalls, ending a streak of losses this week as a heatwave cut demand and pipeline maintenance left the market with nowhere to send excess gas. * Fresh liquefied natural gas (LNG) shipments from Algeria and Qatar converging on Britain left the market facing a potential glut. The shutdown of the Britain-Belgium pipeline link, undergoing maintenance until June 29, meant there was no way to export the surplus to mainland Europe. * British within-day gas price up 4.5 pence or 17.65 percent at 30 pence per therm by 0852 GMT. * Day-ahead gas price 4.30 pence higher at 29.25 pence/therm. * A trader said the rally may prove unjustified and prices could drift back to 25-27 pence on the prompt, adding that fundamentals were little changed except for lower Norwegian flows. * System undersupplied by 4.7 million cubic metres per day (mcm/day) with demand at 159.6 and supply at 154.9 mcm/day, National Grid (LSE: NG.L - news) data showed. The system opened undersupplied by 12 mcm/day. * Forecast demand is much lower than the seasonal norm of 197 mcm/day. * Three LNG tankers are due to arrive in Britain by early July and the next delivery, from Algeria, is scheduled to arrive on Sunday. * Average temperatures for Monday were revised upwards by up to 4.7 degrees Celsius compared with earlier estimates, to around 20.7 degrees, weather data from Thomson Reuters (Dusseldorf: TOC.DU - news) showed. * Norwegian flows through the Langeled pipeline stayed depressed at 7 mcm/day, 2 mcm/day below Thursday levels and 7 mcm/day below Wednesday. * July gas contract up 0.10 pence at 34.60 pence/therm. * Day-ahead gas price at the Dutch TTF hub up 0.32 euro at 15.02 euros per megawatt-hour. * Benchmark Dec-17 EU carbon contract down 0.06 euro at 4.90 euros per tonne.
* Thomson Reuters analyst view: http://emea1.apps.cp.extranet.thomsonreuters.biz/cms/?pageid=united-kingdom-gas (Reporting by Oleg Vukmanovic; Editing by Dale Hudson)